Compositions including reinforcing fibers and a thermoplastic resin are lightweight and excellent in dynamic characteristics, and are therefore widely used in sporting goods applications, aerospace applications and general industrial applications. As reinforcing fibers that are used for these fiber-reinforced thermoplastic resin compositions, metallic fibers such as aluminum fibers and stainless steel fibers; organic fibers such as aramid fibers and PBO fibers; inorganic fibers such as silicon carbide fibers; and carbon fibers are used. From the viewpoint of a balance of specific strength, specific rigidity and lightness, carbon fibers are suitable, and among them polyacrylonitrile-based carbon fibers are particularly suitably used.
For molding a composition including reinforcing fibers and a thermoplastic resin, generally a molding material or a prepreg is produced by combining reinforcing fibers with a thermoplastic resin beforehand, and is used to mold the composition.
Particularly, a molding material processed in a pellet shape can be applied to molding methods excellent in economical efficiency and productivity, such as injection molding and stamping molding, and is useful as an industrial material. In recent years, attempts have been made to enhance performance by controlling arrangement of a matrix resin and reinforcing fibers for the pellet-shaped molding material.
Since a prepreg has good handling characteristics during lamination, a molding method is widely used in which a preform obtained by laminating prepregs is press-molded (molding method in which the preform is defoamed under applied pressure to be shaped).
The method for producing a prepreg is generally a method in which a reinforcing fiber base obtained by unidirectionally arranging or weaving continuous reinforcing fibers is impregnated with a resin to produce a prepreg. A molded article obtained by molding the prepreg using continuous reinforcing fibers has excellent dynamic characteristics, but is unsuitable for molding to a complex shape because reinforcing fibers are used in the form of a continuous body, and therefore a molded article using discontinuous reinforcing fibers has also been proposed.
In recent years, fiber-reinforced thermoplastic composite materials have increasingly received attention, and come into use in a variety of diverse applications. Molded articles excellent in dynamic characteristics have been required, and from an industrial viewpoint, higher economical efficiency and productivity have become necessary. For example, further lightness and economical efficiency have been desired, and polyolefin resins that are lightweight, particularly polypropylene resins, have come into use as a matrix resin.
However, the polypropylene resin is poor in interfacial adhesion with reinforcing fibers, and it is difficult to obtain a molded article excellent in dynamic characteristics. In particular, when fibers having poor surface reactivity, such as carbon fibers, are used as reinforcing fibers, it is particularly difficult to obtain a molded article excellent in dynamic characteristics. Thus, attempts have been made to improve interfacial adhesion between carbon fibers and polypropylene by surface modification or sizing treatment of carbon fibers, and modification of a matrix resin by addition of modified polypropylene.
Patent Document 1 discloses a molding material in which a high-molecular-weight thermoplastic resin is arranged so as to be in contact with a composite including a low-molecular-weight thermoplastic polymer and continuous reinforcing fibers. In the molding material, a low-molecular-weight material is used to impregnate continuous reinforcing fiber bundles while a high-molecular-weight material is used for a matrix resin to secure all of economic efficiency, productivity and dynamic characteristics. It is shown that when the molding material is molded by the injection molding method, the molded article obtained can have an increased fiber length of reinforcing fibers as compared to conventional ones, so that both good dynamic characteristics and excellent external appearance quality can be achieved.
Patent Document 2 proposes that a seal material having more isotropic characteristics is obtained by dispersing reinforcing fibers in the form of bundles. Patent Document 3 proposes that a seal material excellent in dynamic characteristics is obtained by uniformly dispersing carbon fibers.
Patent Document 4 proposes a prepreg and a preform which include reinforcing fibers having a specific fiber length and a specific two-dimensional orientation angle, and have a specific thickness, and it is disclosed that by using the prepreg, molding to a complex shape is possible, and a molded article extremely excellent in isotropy and dynamic characteristics can be obtained. Patent Document 5 proposes a prepreg having a linear notch over the entire surface of a prepreg layer, the notch forming an angle in a range of 2 to 25° in terms of an absolute value with respect to reinforcing fibers, wherein substantially all the reinforcing fibers are separated by the notch, and the fiber length of the reinforcing fibers separated by the notch is in a range of 10 to 100 mm, and it is disclosed that the prepreg is excellent in shape followability to a complex shape and can be molded in a short time, and the resulting molded article has excellent dynamic properties applicable to a structural material, reduced variation thereof and excellent dimensional stability. Each of Patent Documents 4 and 5 shows an example of using acid-modified polypropylene in combination when a polypropylene resin is used as a matrix resin.
Patent Document 6 discloses carbon fibers sizing-treated with a polyfunctional compound, and a polypropylene resin composition using a terpene resin, and it is shown that the resin composition is excellent in moldability and interfacial adhesion, so that a molded article excellent in flexural characteristics and impact resistance can be obtained.
Patent Documents 7 and 8 show that by adding maleic anhydride-modified polypropylene, a matrix resin is modified, and interfacial adhesion between carbon fibers and polypropylene is improved.
Patent Document 9 shows that by adding a polyolefin resin modified with a polycarbodiimide group for further improving dynamic characteristics, and a matrix resin is modified, dispersibility of carbon fibers is improved, so that a molded article excellent in flexural characteristics and impact resistance can be obtained.